Using rhesus monkeys equipped with (1) gastric cannulas (to obtain sham-feeding), (2) duodenal cannulas (to introduce food directly into the intestine), (3) gallbladder cannulas (to measure pressure changes) and (4) intraportal venous catheters (to infuse exogenous cholecystokinin or its fully active synthetic octapeptide and to withdraw blood samples), we will compare the amounts of exogenous and endogenous cholecystokinin (CCK) required to produce gallbladder contractile responses with the amounts required to produce satiety. These comparisons will indicate whether the satiety action of CCK is physiological or pharmacological. We will determine whether CCK is necessary for satiety in the rat by attempting to block the satiety action of (1) CCK and its synthetic octapeptide during sham-feeding (2) intestinally-perfused food during sham-feeding and (3) normally ingested food, using the low-activity desulfated octapeptide analogue SQ19265 as the blocking agent. We will identify the peripheral and/or central sites of action of CCK in the rat by a variety of methods: (1) parasympathetic and sympathetic gut denervations, (2) intravenous infusions of other gut hormones besides CCK, singly and in combination, (3) radio-active labelling and tracing of CCK and the octapeptide, (4) intracerebral injections of CCK and the octapeptide, and (5) electrical brain lesions.